The present invention relates generally to image-forming devices, and more particularly to a development device and development method for use with an electrophotographic image-forming device. The present invention also relates, for example, to a development method using a nonmagnetic monocomponent developing agent, development roller, and a blade that regulates a layer thickness of the nonmagnetic monocomponent developing agents on the development roller, and to a method of forming a layer thickness of the nonmagnetic monocomponent developing agents, on a development device wherein the development device has the development roller and the blade, and an electrophotographic image-forming device including one or more of these elements. However, it is to be understood that the scope of application of the present invention is not limited to devices using the nonmagnetic monocomponent developing agent.
The xe2x80x9cnonmagnetic monocomponent developing agentxe2x80x9d is a single component developing agent that is not magnetized and includes no carrier. The xe2x80x9celectrophotographic image-forming devicexe2x80x9d is an image-forming device employing the Carlson process described in U.S. Pat. No. 2,297,691, as typified by a laser printer, and denotes a nonimpact printer that provides recording by depositing developing agents as a recording material on a recordable medium (e.g., printing paper, and OHP film).
The nonmagnetic monocomponent developing agent commonly includes the toner having a relatively high volume resistivity (e.g., at 300 Gxcexa9xc2x7cm, etc.). In addition, the toner, since it basically carries no electric charge, needs to be charged by the triboelectricity or charge injection in the development device.
With the recent development of office automation, the use of electrophotographic image-forming devices for computer output devices, facsimile units, photocopiers, etc. have spread steadily. Particularly, a laser printer as one example of the electrophotographic image-forming devices features good operability, usability for a wide range of media, high cost efficiency, and high printing quality, whereby a further improvement in high-quality and high-speed printability will be expected in future. The electrophotographic image-forming device generally includes a photoconductive insulator (photoconductor; photosensitive drum), and follows the procedural steps of charging, exposure to light, development, transfer, fixing, and other post-processes.
The charging step uniformly electrifies the photosensitive drum (e.g., at xe2x88x92700 V). The exposure step irradiates a laser beam, or the like, on the photosensitive drum, and changes the electrical potential at the irradiated area down, for example, to xe2x88x9250 V or so, forming an electrostatic latent image. The development step electrically deposits developing agents onto the photosensitive drum using, for example, the reversal process, and visualizes the electrostatic latent image. The reversal process is a development method that forms an electric field by a development bias in areas where electric charge is eliminated by exposure to light, and deposits the developing agents having the same polarity as uniformly charged areas on the photosensitive drum by the electric field. The transfer step forms a toner image corresponding to the electrostatic latent image on a recordable medium. The fixing step fuses and fixes the toner image on the medium using heat, pressure, or the like, thereby obtaining a printed output. The post-processes may include charge neutralization and cleaning on the photosensitive drum from which toner has been transferred out, a collection and recycle and/or disposal of residual toner, etc.
The developing agent for use with the aforementioned development step can be broadly divided into a monocomponent developing agent using toner and a dual-component developing agent using toner and a carrier. The toner may include a particle prepared, for example, in such a manner that a colorant, such as a dye and carbon black, or the like, is dispersed in a binder resin made of synthetic macromolecular compound, and then is ground into a fine powder of approximately 3 through 15 xcexcm. A usable carrier may include, for example, an iron powder or ferrite bead of approximately 100 xcexcm diameter. The monocomponent developing agent advantageously results in (1) simple and miniature equipment for the development device due to eliminating carrier deterioration, a toner density control, mixing, and agitation mechanisms, and (2) no residual waste, such as a carrier in used toner.
The monocomponent developing agent may be further classified into a magnetic monocomponent developing agent that includes a magnetic powder in toner, and a nonmagnetic monocomponent developing agent that does not include the same. However, the magnetic monocomponent developing agent is disadvantageous in: (1) low transfer performance due to the high content of low electrical resistant magnetic powder which hinders the increased electric charge amount; (2) bad colorization due to its low transparent, black-color magnetic powder; and (3) low fixing performance due to the magnetic powder which requires high temperature and/or high pressure, thereby increasing a running cost. Accordingly, the nonmagnetic monocomponent developing agent without these disadvantages is expected to be in increasing demand in future.
The development method employing the nonmagnetic monocomponent developing agent is divided into two development methods: one is a contact-type development method that deposits developing agents on the photosensitive drum by bringing the development roller carrying the developing agents into contact with the photosensitive drum; and the other is a jumping development method (noncontact-type development method) that provides a certain gap (e.g., of about 350 xcexcm) between the development roller and the photosensitive drum to space them from each other, and flies the developing agents from the development roller to, and deposits the same onto, the photosensitive drum.
It is significant for the development process employing the nonmagnetic monocomponent developing agent to ensure a sufficient image density by controlling the amount of toner conveyed from the development roller to the photosensitive drum. Thus, it is very important to form a specified toner layer while controlling its thickness on the development roller. As a typical method for regulating a toner layer thickness, it has conventionally been proposed to provide a blade (restriction blade) in contact with the development roller to maintain the layer thickness uniform.
Referring now to FIG. 8, a description will be given of a contact-type development device 10 using a nonmagnetic monocomponent developing agent. FIG. 8 is a schematic sketch of a principal part of the conventional development device 10 for explaining a bias applied to the development device 10. As shown in FIG. 8, the development device 10 includes a development roller 12, a reset roller 14, and a blade 16. The development roller 12 adsorbs onto a surface thereof charged toner as a thin layer, and conveys the toner to a development area in contact with the photosensitive drum. The development roller 12 is connected with a bias power supply (not shown) that applies a development bias Vb. The reset roller 14, which is also called a supply roller or application roller, contacts the development roller 12 and serves to supply toner to the development roller 12. Further the reset roller 14 also serves to scrape off and remove the toner unused for the development and remaining on the development roller 12. As shown in FIG. 8, a reset bias Vr is applied to the reset roller 14.
The blade 16 is brought into contact with the development roller 12, and serves to regulate the toner layer to a uniform thickness. In order to avoid damaging the blade 16 and the development roller 12 by mitigating the accuracy in contact pressure required at the contact portion therebetween, it is so devised that one of them is made of an elastic body when the other is made of a rigid body. For example, when a metal member, namely a rigid member, is used for the development roller 12, the toner layer may be regulated by bringing the blade 16 made of an elastic body, such as rubber, into contact with the development roller 12. On the other hand, when a member made of an elastic body, such as rubber, is used for a surface of the development roller 12, the toner layer may be regulated by bringing an end portion or non-end portion of the blade 16 made of metal into contact with the development roller 12. However, the blade 16 made of an elastic body, such as rubber, would be abraded (worn) by repeated development operations, and thus the number of sheets that can be printed would disadvantageously be limited to ten through twelve thousand sheets. Therefore, the use of the metal blade 16 resistant to abrasion has recently received attention.
The metal blade made of stainless steel (SUS) can inject charges into toner by a blade bias Vdb applied to the blade, as shown in FIG. 8. According to the conventional development device, the reset bias Vr and the blade bias Vbd share a bias power supply (not shown) in order to avoid an increase in costs with the increasing number of power supplies. The bias voltages applied to each element 12 through 16 may, for example, be set as follows: the development bias Vb is xe2x88x92300 V; the reset bias Vr is xe2x88x92400 V; and the blade bias Vbd is xe2x88x92400 V.
A toner layer on the development roller, if too thin, would result in a low and uneven image density, while, if too thick, would increase a proportion of oppositely charged or low charged toner, thereby producing a fog in a no-image area (i.e., undesirably coloring with the toner an area which has no image and is therefore expected to be white clarity). Thus, the blade 16 is required to form a toner layer having an appropriate thickness.
In development operation, the toner is charged (e.g., negatively) through sliding friction among the reset roller 14, the blade 16, and the development roller 12. The negatively charged toner thereafter is fed onto a surface of the development roller 12 by the reset roller 14, and deposited thereon by electrostatic adsorption. Subsequently, the toner layer on the development roller 12 is leveled using the blade 16 to form a thin layer having a uniform thickness of about 10 xcexcm through 40 xcexcm. The toner is conveyed from the photosensitive drum to the development roller 12, and adsorbed to an electrostatic latent image on the photosensitive drum with the electrical force of attraction using a predetermined voltage applied to a development area. Consequently, the latent image is visualized and developed. Next, the residual toner unused for the development and remaining on a no-image area of the development roller 12 in which no latent image is formed is removed by the reset roller 14 from the development roller 12. The development process repeats a series of these operations.
However, the conventional contact-type development method employing a nonmagnetic monocomponent developing agent disadvantageously produces images having a variety of image quality according to the development conditions. The present inventors first elaborately studied the causes that would deteriorate the image quality, and resultantly found out that the image quality depends upon a change of the toner charge amount.
In the aforementioned development process, a toner charge amount depends on the charge injection by the blade 16. When a toner layer is formed on the development roller 12 but not developed, the toner layer is configured to be flaked off from the development roller 12 by the reset roller 14, but actually, a considerable amount of the toner is left and conveyed to, and brought into contact with, the blade 16 again, and additional charges are thereby injected, increasing the toner charge amount. Consequently, the toner is separated from the development roller 12, which may make it difficult for the toner to be adsorbed onto the photosensitive drum, and thus produce image retention (area-to-area variations in image density). In addition, when a solid image (an image of which an entire area to be printed is filled in) is developed, an image density on a first printed output would disadvantageously become low, and thereafter the image density would increase as the cycle of rotation of the development roller 12 proceeds.
As the number of printed sheets increases, the toner is degraded, and a charging capability (charge amount) of toner decreases; therefore negatively charged toner deposited on the development roller increases so as to compensate for a potential difference between the metal blade 16 (at xe2x88x92400 V) and the development roller 12 (at xe2x88x92300 V). Consequently, the amount of charges applied to a unit amount of toner decreases (or charge injection effect decreases), and thus the toner charge amount further decreases, disadvantageously producing a fog.
The above-described disadvantages derived from a variation of the toner charge amount would be eliminated in principle by equalizing the potentials of the development roller 12 and the blade 16, and dispensing with the charge injection from the blade 16 to the toner. In other words, the development bias Vb and the blade bias Vbd may be adjusted to the same potential, and according to this adjustment, almost all of the toner is charged only by friction between each toner particle, or triboelectricity by the reset roller 14, and thus is not charged by the charge injection into the toner. However, a surface of a toner layer charged by friction comes in contact with the blade 16, and thus is further charged by friction. Accordingly, an area of the surface of the toner layer in contact with the blade 16 has a higher potential than the blade 16 by the amount of potentials increased by friction with the blade 16, and thus a potential difference occurs, so that an oppositely charged toner is produced. The oppositely charged toner denotes toner having a charge opposite in polarity to a charge that works effectively in the development process. As a result, even if the development roller 12 and the blade 16 have the same potential, the oppositely charged toner exerts an influence on toner layer, and increases the susceptibility to fogging.
Accordingly, it is an exemplified general object of the present invention to provide a novel and useful development device, development method, and image-forming device, in which one or more of the above-described conventional disadvantages are eliminated.
Another exemplified and more specific object of the present invention is to provide a development device, development method, and image-forming device that can more stably form a high-quality image than was previously possible by a cost-efficient means for stabilizing a toner charge amount, and forming a toner layer having reduced dependence on the toner charge amount.
In order to achieve the above objects, the development device as one exemplified embodiment of the present invention comprises: a development roller, a surface of which is made of electrically resistant material; a blade that comes in contact with the development roller so as to form a layer of developing agents, has a predetermined thickness on the development roller, and possesses electrical conductivity; a bias power supply that applies a bias to the development roller and the blade; and a resistance provided between the blade and the development bias supply to establish electric connection therebetween. According to this development device, the blade is connected with the resistance, and thus can discharge the excessively charged developing agents. Therefore, the charge amount of the developing agents becomes stable, and the toner layer formation becomes preferable.
The development method as one exemplified embodiment of the present invention comprises: a reset roller to which a current voltage is applied feeding developing agents to a development roller to which a current voltage is applied, by utilizing a potential difference; bringing a blade to which a current voltage is applied into contact with the development roller, and forming a uniform layer of the developing agents charged by triboelectricity, the blade being connected with a resistance; a development process of feeding the developing agents from the development roller disposed in contact with the photosensitive drum to the photosensitive drum, and visualizing an image with the developing agents on the photosensitive drum; and the reset roller collecting residual developing agents on the development roller utilizing a potential difference, wherein the resistance ranges between 50 kxcexa9 and 100 Mxcexa9. This development method has the same actions as the above development device.
The image-forming device as one exemplified embodiment of the present invention comprises a photosensitive drum; a charger that charges the photosensitive drum; an exposure part that exposes the surface of the photosensitive drum charged by the charger to light, and forms an electrostatic latent image; a development device that develops the surface of the photosensitive drum exposed to light, and visualizes the electrostatic latent image into a toner image; and a transfer part that transfers the toner image onto a recordable medium, wherein the development device comprises: a development roller a surface of which is made of electrically resistant material; a blade that comes in contact with the development roller so as to form a layer of developing agents, and has a predetermined thickness on the development roller, and possesses electrical conductivity; a bias power supply that applies a bias to the development roller and the blade; and a resistance provided between the blade and the development bias supply to establish electric connection therebetween. Alternatively, the above development device comprises a development roller a surface of which is made of electrically resistant material; a blade that comes in contact with the development roller so as to form a layer of developing agents, and has a predetermined thickness on the development roller; a bias power supply that applies a bias to the development roller and the blade; and an elastic resistant material that is directly electrically connected with the development bias power supply, and makes up the blade. This image-forming device includes the above development device, and thus exerts the same action as the development device.
Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings.
FIG. 1 is a partial sectional view of a principal part of a development device and an image-forming device as one exemplified embodiment of the present invention.
FIGS. 2 and 2(a) are schematic sketches of principal parts of embodiments of the development device shown in FIG. 1 for explaining a bias applied to the development device.
FIG. 3 is a schematic equivalent circuit diagram for electrically explaining the development device shown in FIG. 1.
FIG. 4 is a conceptual diagram for showing a solid image density for every cycle of rotation of the development roller.
FIG. 5 shows a relationship between a cycle of rotation of the development roller and a solid image density in resistance R.
FIG. 6 is a graph for showing a relationship between the number of printed sheets and a solid image density.
FIG. 7 is a graph for showing a relationship between the number of printed sheets and fogging on the drum.
FIG. 8 is a schematic sketch of a principal part of a conventional development device for explaining a bias applied to the development device.